This application is for the K99/R00 Pathway to Independence award. I am currently a senior postdoctoral fellow in the Fishell lab at the NYU-School of Medicine and I have an extensive background in molecular biology and mouse genetics. My career development plan is designed to acquire theoretical and practical training in electrophysiology under the guidance of Drs. Gord Fishell and Bernardo Rudy. In addition, the development plan is aimed at strengthening my presentation, grant-writing and teaching skills. A postdoctoral advisory committee (PAC) will oversee my progress and assess my readiness to enter the job market and make the transition to an independent laboratory. Finally, I will take courses and workshops to develop a background in neurological and neuropsychiatric disease entities with the goal of improving my ability to consider my research findings in the context of potential links to human pathologies. The K99 portion of the award would take place within the Smilow Neuroscience Program at NYU-School of Medicine. This program, in combination with the larger Neuroscience community at NYU (Center for Neural Science and Skirball Institute), will provide a superb academic environment in which to complete my training and successfully transition to an independent academic position. Scientific Abstract: Recent experimental evidence has revealed that intrinsic genetic programs endow GABAergic interneurons with an early subtype identity. It is also known that interneurons participate in correlated network activiy during development. Indeed, my previous work indicates that the radial migration and morphological development of calretinin and reelin but not vasoactive intestinal peptide interneurons, the major subtypes derived from the caudal ganglionic eminence (CGE), are activity-dependent. Furthermore we have found that glutamatergic drive is essential for mediating the activity required for the proper development of axons and dendrites towards the end of the first postnatal week. However, the mechanisms by which activity regulates interneuron maturation are not fully understood. This proposal is aimed at revealing the identity of the neuronal types that provide interneurons with the neurotransmitters necessary for laminar targeting, and for the proper formation of axons and dendrites (Aim 1). In addition, this project will explore the role of glutamate receptors in morphological development (Aim 2). Finally, a long-term aim of this research plan is to describe the connectivity pattern of developing interneurons as they integrate into cortical circuits, and to assess how neuronal activity may regulate the process by which this pattern is generated (Aim 3). A variety of neuronal cohorts populate the cortex during the first postnatal week, when activity-dependent maturation of interneuron subtypes takes place. Glutamatergic cell cohorts present during this time include Cajal-Retzius cells, glutamatergic transient cells, subplate cells and pyramidal cells. Due to thei spatial and temporal distribution, these cohorts are well suited to provide interneurons with the glutamatergic drive that is fundamental for their morphological development. Glutamate release from each individual cohort will be genetically blocked to assess the impact of these populations on interneuron maturation (Subaim 1a). GABAergic transmission is also prominent at early stages of cortical development and may contribute to laminar targeting. To assess the role of GABA in radial migration, GABA receptors will be blocked pharmacologically (Subaim 1b). While our previous experiments have indicated a requirement for glutamate in morphological development, the mechanism responsible for activity-sensitive maturation is not understood. Due to the developmental role of NMDA receptors, our experiments will focus on the study of these ionotropic receptors during interneuron development. The cell-autonomous consequences of NMDA receptor removal in CGE interneuron will be assessed. Our analysis will also include the study of the signaling pathways operating downstream of these receptors (Aim 2). After interneurons undergo migration and develop characteristic morphologies, they integrate into cortical circuits. However, the identity of synaptic inputs to specific subsets of CGE interneurons are unknown. Monosynaptic viral tracing techniques will be used in combination with in utero electroporation to reveal the pattern of connectivity of maturing interneurons (Subaim 3a). In addition, our experiments will assess the impact of perturbing neuronal activity on the integration of interneurons into nascent cortical circuits (Subaim 3b). The experiments in this grant proposal will be carried out in vivo in the mouse somatosensory cortex. The principles that will emerge from these studies, however, are expected to apply to other regions of the cortex as well. A better understanding of interneuron development and GABAergic circuit formation over a potentially broad set of cortical areas is likely to contribute to our understanding of the pathogenesis of diseases in which interneuron defects are thought to play a role. In addition, the experimental approach presented in this proposal exemplifies the advantage of interdisciplinary collaboration within the field of neurobiology. Indeed, it is my conviction that the integration of both the conceptual approach and experimental techniques from two particular subfields, developmental genetics and electrophysiology, will continue to advance our understanding of CNS function and pathology.